In this paper, the results of an experimental study of laminar natural convection heat transfer and fluid flow in horizontal annuli between a cylindrical envelope and its inner concentric octagonal heated cylinder are presented. Two octagonal cylinders are investigated: one with a complete surface and the other with two horizontal slots on the top and bottom surfaces. The ratio of the slot width W to H is 0.072. Air is used as the working fluid. The range of Rayleigh number is 2.1×102–1.58×106 for the unslotted case and 1.2×102–1.5×106 for the slotted case. The average heat transfer correlations for the two cases are provided. The results show that the heat transfer intensity of the unslotted octagon is slightly weaker than that in a cylindrical annulus, while for the slotted case, the overall heat transfer enhancement may be as high as 74 percent. The smoke technique is used to visualize the flow patterns. A series of photographs of the flow patterns are provided, which enhances our understanding of the mechanism of heat transfer enhancement for the slotted octagonal case.

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